1. Field of the Invention
The present invention relates to a flat brushless motor utilizable in a portable communication device or the like.
2. Description of the Prior Art
It has been a problem that a conventional communication device, such as a pocket bell, only alerts a particular person, but the audible signals are in fact transmitted to other people on the occasion of a meeting, conference, etc.
As such, a device designed to alert a particular person only by way of vibrations has been developed, and a brushless motor has been utilized as a source of the vibrations.
One example of the flat brushless motor of rotary type will be described hereinbelow with reference to the accompanying drawings.
FIG. 5 illustrates the conventional flat brushless motor of rotary back-yoke type.
Reference numeral 1 represents a flat annular magnet magnetized so as to have N- and S-poles in a circumferential direction in which magnetic fluxes are generated; reference numeral 2 represents a rotor frame for shielding the magnetic fluxes produced by the magnet 1; reference numeral 3 represents a bracket for generating a thrust force in an axial direction by magnetic fluxes leaking out from the rotor frame 2; reference numeral 3a represents a carrier made of insulating material for holding coils; reference numeral 4 represents a plurality of flat coils interlinking the magnetic fluxes generated by the magnet 1 for generating a rotating force when an electric current runs thereacross; reference numeral 5 represents a back-yoke for enclosing a magnetic circuit of main magnetic fluxes produced by the magnet 1; and reference numeral 6 represents a weight for causing the motor to generate unbalanced vibrations. Reference numeral 7 represents a shaft of the motor; reference numeral 8 represents a sleeve bearing; and reference numeral 9 represents a rotor boss rigidly mounted on the shaft 7 and to which the rotor frame 2 and the back-yoke 5 are secured.
FIG. 6 illustrates the shape of the weight 6.
With respect to the conventional flat brushless motor of rotary back-yoke type constructed as hereinabove described, the operation thereof will be described hereinafter.
Assuming that magnetic fluxes are generated from the magnet 1, the magnetic flux developing downwards is shielded by the rotor frame 2 and the bracket 3. Because of this, the main magnetic flux develops in a direction towards the coil 4.
If an electric current is allowed to flow across the coils 4 under this condition, a magnetic field is developed by the current and a rotating force is produced by the relationship with the magnetic flux of the magnet 1. Also, a magnetic circuit is formed by the magnetic flux towards the back-yoke 5 to thereby increase the efficiency or the motor. The weight 6 with its unbalanced shape is fixed to the shaft 7, so that the motor is allowed to produce an unbalanced rotation when the weight 6 is rotated.
However, in the conventional construction, since a portion corresponding to the weight 6 renders the thickness of the motor to be large, there are problems in achieving a flatter motor, and the number of manufacturing steps is increased, as well as the number of component parts.